This invention pertains to vibration damping of structures that release vibrational energy through their surfaces, and more specifically to compositions for vibration damping of such structures that include a visco elastic damping layer to convert vibrational energy to stress energy and/or strain energy and dissipate this energy as heat. Thus, the visco elastic damping layer dissipates vibrational energy in the form of heat as it is subjected to vibratory motions. The dissipation of heat from the visco elastic damping layer, i.e., the efficiency of the visco elastic damping layer, is directly proportional to the strain levels generated in this damping layer during the vibratory motion. It is thus desirable to maximize the generation of total strain energy in the damping layer. In operation, as the primary structure deforms due to vibratory motion, the damping layer likewise deforms. In this manner, strain energy is incurred which is subsequently dissipated as heat. This dissipation of heat removes energy from the vibratory motion of the primary structure, resulting in less remaining vibratory energy and a faster decay of the vibratory motion compared to an undamped primary structure.
The primary structure referred to above includes any structure having a surface through which vibrational energy is released. More specifically, the term primary structure includes exterior walls, interior walls, foundations, floors, ceilings of buildings, etc., and other structures where noise reduction is desired, as well as structures of transportation devices such as aircraft, automobiles, and ships, where for example noise reduction and/or increased material fatigue levels are desired.
In the past, a common way of vibration damping has been "constraint layer" damping in which damping tape comprised of a layer of visco elastic material is attached to the surface of the structure whose vibration is to be damped and a thin constraining layer formed of, for example, aluminum foil, is attached to the other side of the damping layer. The constraining layer may have a high elastic modulus such that it is stiff in longitudinal extension but bends readily. As the primary structure undergoes bending or longitudinal deflection during the vibratory motions, the damping layer is sheared between the primary structure and the constraining layer. Thus, vibrational energy is converted into shear energy which, in turn, is dissipated as heat by the damping layer. In this manner, vibrational energy is further reduced when compared to the use of a damping layer without a constraint layer. The use of damping tape, however, adds a large amount of weight to the damped primary structure, and is very labor intensive to apply to the primary structure.
U. S. Pat. No. 4,425,980, issued to Miles discloses another type of vibration damping composition in which beam dampers comprising a stiff, lightweight, elongate beam and a layer of visco elastic material located along an attachment flange of the beam are employed. The flange of the beam is attached by the layer of visco elastic material to the skin of the structure whose vibrations are to be damped. The beam acts as a constraining element for the visco elastic attachment layer. The beam is oriented such that it is stiff in a plane transverse to the plane of the structure skin, resulting in thickness deformation of the layer of the visco elastic material (rather than shear deformation) and conversion of the vibration energy into heat. The above vibration damping composition suffers from two problems. First, the beam required by the above invention significantly adds to the required thickness of the damped structure. Additionally, the overall weight of the damped structure is markedly increased.
Additional vibration damping techniques are discussed in the above Miles patent, which is incorporated herein by reference.
A need thus exists for a lightweight vibration damping composition lacking stiffening beams or members that add excessive weight, cost, and complexity to the composition. A need also exists for a vibration damping composition of the above type in which an increased amount of vibratory energy from the primary structure is converted into strain energy by the vibration damping composition, and is dissipated as heat by the presence of either non-homogeneities in the damping layer or discontinuities between the primary structure and the damping layer.